Nearly a year after NASA trumpeted Voyager 1’s departure from the sun’s protective bubble, two mission scientists argue that the spacecraft never left. Many astronomers are doubtful about the assertion, but the debate illustrates that the transition from solar bubble to interstellar space is not clear-cut.

“My tendency is to think we are out in interstellar space, but I’m not completely convinced,” says Eric Christian, an astrophysicist at NASA Goddard Space Flight Center in Greenbelt, Md., who is not on the Voyager team. “I don’t blame skeptics for looking at other explanations.”